1. Field of the Invention
The present invention relates in general to the field of hologram production and, more particularly, to hologram production using pulsed lasers.
2. Description of the Related Art
One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms in holographic recording materials without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms can be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is typically directed through a spatial light modulator (SLM) displaying a rendered image and then interfered with a reference beam. Examples of techniques for one-step hologram production can be found in U.S. patent application Ser. No. 09/098,581, entitled “Method and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,” naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17, 1998, (“the '581 application”; now U.S. Pat. No. 6,330,088) which is hereby incorporated by reference herein in its entirety.
In general, the hologram production devices (often referred to as “hologram recorders”) described in the '581 application can use either continuous-wave (CW) or pulsed lasers as the coherent light source for the object and reference beams used to create interference patterns. Hologram recorders often use CW lasers because they are more commercially available, their output intensity is typically easier to control, and because it is typically easier to find a CW laser that will produce output at a single desired frequency. Moreover, many of the preferred holographic recording materials, such as photopolymerizable compositions, dichromated gelatin, and silver halide emulsions, are particularly suited for use with CW laser sources.
Nevertheless, the use of CW lasers in hologram recorders does present certain limitations. Chief among those limitations is the relatively low output power of CW lasers which causes the hologram recorder to use relatively long exposure times (e.g., tens of milliseconds) for each hogel. During those exposure times, the entire hologram production system is particularly susceptible to mechanical vibration. Great effort is expended to reduce or eliminate the mechanical vibrations. Hologram recorders are typically located far away from sources of environmental vibration, such as outside traffic, building vibration, mechanical equipment, common appliances, human motion, acoustic noise, plumbing turbulence and air flow. Special devices, such as vibrationally-isolated optics tables, are typically used where environmental vibration sources cannot be sufficiently reduced or eliminated. Such devices and techniques add cost and complexity to hologram production systems. Moreover, to help ensure a stable hogel recording environment, a step-repeat approach is often adopted in translating the holographic recording medium. Consequently, additional settling time (on the order of tens of milliseconds as well) is introduced into the recording process. The cumulative recording and settling times prolong the hologram production process, making it more expensive and in some cases impractical for certain applications. Moreover, the mechanical systems used to step the system, bring (or allow) the system to come to a stop, and repeat can be very complex.
Using pulsed lasers in hologram production devices can mitigate or solve many of the aforementioned problems associated with CW laser use. Due to the different physics of pulsed laser operation, a small frame pulsed laser is able to generate higher light intensity than its CW counterparts. For example, small frame frequency doubled Nd:YAG pulsed lasers can generate 1.1 mJ of energy during a 35 ns long pulse at 532 nm. This corresponds to approximately 31.4 kW of power during the pulse. In contrast, a typical CW Nd:YAG laser produces approximately 4 W of power. Because high exposure intensity is possible using pulsed lasers, the required exposure time to generate a hologram can be reduced significantly. While tens of milliseconds is typically needed for CW laser hologram recording, the tens of nanoseconds pulse duration of a pulsed laser is adequate for pulsed laser hologram recording. Decreasing the exposure time by six orders of magnitude means that the frequencies of both the movement of the hologram recorder components and environmental vibration are too low to generate any noticeable effect on interference pattern generation. The mechanical stability requirements restricting the CW laser based hologram recorder are completely relaxed. Consequently, the recorder design can be significantly simplified and the cost of the hardware is reduced.
Despite the advantages of using pulsed lasers in hologram production devices, the holographic recording materials typically used may not provide adequate results when used with pulsed lasers. For example, photopolymerizable compositions (photopolymers) are among the most preferable holographic recording materials because of the image recording capabilities and their relative ease of use. Photopolymers include a wide range of materials that undergo physical, chemical, or optical changes through selective polymerization when exposed to light. Typically, photopolymers include a monomer or a crosslinkable polymer, a sensitizer or photoinitiator, and a binder or liquid to hold the components. Changes in the photopolymer's refractive index, transparency, adhesion, and/or solubility differentiate light and dark regions when these materials are exposed to an activating light source. Photopolymers capable of recording volume phase holograms include those developed by Canon Incorporated (based on polyvinyl carbazole), Polaroid Corporation (based on polyethylene amine/acrylate), and E. I. du Pont de Nemours and Company (based on polyvinyl acetate and polymethyl methacrylate). Those having ordinary skill in the art will readily recognize that a variety of different photopolymer compositions can be used in the practice of the inventions described herein. Nevertheless, preferred photopolymer films are provided by E. I. du Pont de Nemours and Company under the trade designations, for example, OmniDex™ 706, OmniDex™ 801, HRF-800X001-15, HRF-750X, HRF-700X, HRF-600X, and the like.
Holograms recorded in photopolymer films using single laser pulses from pulsed lasers are known to be of generally poorer quality as compared to holograms recorded in photopolymer films using CW lasers. For example, in V. N. Mikhailov, K. T. Weitzel, V. N. Krylov, and Urs P. Wild, “Pulse Hologram Recording in DuPont's Photopolymer Films,” Practical Holography XI, Proc. SPIE, vol. 3011, pages 200-202, Feb. 10-11, 1997, (the Mikhailov reference) which is hereby incorporated by reference herein in its entirety, it was demonstrated that a hologram recorded with a 25 ns pulse from a YLF-Nd Q-switched laser (0.25 J/cm2 intensity) had a peak diffraction efficiency of approximately 6.5%, while a hologram recorded for 5 seconds using a comparable intensity argon-ion CW laser had a peak diffraction efficiency of approximately 92%. Diffraction efficiency is a typical measurement of the quality of a recorded hologram and is based on the ratio of diffracted light intensity to input light intensity (usually neglecting Fresnel reflection and absorption in the holographic recording material).
The Mikhailov reference goes on to demonstrate that holograms with larger diffraction efficiencies can be recorded using pulsed lasers if the photopolymer film is pre-illuminated. Specifically, the Mikhailov reference demonstrates that pulsed laser recorded holograms can have diffraction efficiencies of approximately 40% and 75% when the photopolymer film is pre-illuminated using a pulse from the pulsed laser and filtered incoherent light, respectively.
Accordingly, it is desirable to have improved systems and methods for using pulsed lasers to produce holograms and particularly holographic stereograms. Such improved systems and methods would provide high-quality recorded holograms while allowing the hologram production systems to take full advantage of the use of pulsed lasers.